Intraluminal catheter assemblies are employed to diagnose and/or treat abnormalities within the human vasculature. A typical intraluminal catheter assembly includes a distally mounted operative element, such as, e.g., an ablation electrode, which is in electrical communication with a proximally located operative element, such as, e.g., an RF generator. Currently, intraluminal catheter assemblies include elongate catheter bodies in which internal lumens are extruded for the purpose of routing transmission lines between the distally mounted operative element and the proximally located operative element.
Often, intraluminal catheter assemblies support multiple distally mounted operative elements, thereby providing the physician with a single multi-functional platform. Because the radius of the catheter body must be small enough to be transported through the vasculature, however, the size and number of internal lumens which can be extruded through the catheter body becomes a critical factor, thereby limiting the amount, combination and/or performance of distally mounted operative elements supported by these catheter assemblies.
For example, as shown in FIG. 12, a typical ultrasonic imaging/ablation catheter assembly 300 includes an elongate catheter body 302 with distally mounted ablation electrodes 304 and a distally disposed rotatable ultrasonic transducer 306 to allow a physician to more easily image and ablate abnormal vasculature tissue.
The ablation electrodes 304 are electrically coupled to a proximally disposed RF generator (not shown) via transmission lines 308, which are routed through a first internal lumen 310 extruded within the catheter body 302. Operation of the RF generator transmits radio frequency electrical energy through the transmission lines 308 to the ablation electrodes 304, which in turn emit RF energy into the vasculature tissue adjacent the ablation electrodes 304.
The ultrasonic transducer 306 is mounted in a transducer housing 312 disposed within the catheter body 302. The ultrasonic transducer 306 is mechanically and rotatably coupled to a proximally disposed drive unit (not shown) via a drive cable 314 rotatably disposed within a second extruded internal lumen 316. The ultrasonic transducer 306 is electrically coupled to a proximally disposed signal transceiver (not shown) via a transmission line (not shown) disposed within the drive cable 314. Operation of the drive unit rotates the drive cable 314, and thus the ultrasonic transducer 306, with respect to the catheter body 302. Simultaneous operation of the transceiver alternately transmits and receives electrical energy to and from the ultrasonic transducer 306 via the drive cable disposed transmission line, thereby providing the physician with 360.degree. imaging of the vasculature tissue adjacent the ultrasonic transducer 306.
The catheter assembly 300 is configured to allow the drive cable 314 and distally mounted ultrasonic transducer 306 to be "back loaded" (i.e., inserted or retracted) through the second interior lumen 316. The size of the ultrasonic transducer 306, and thus the integrity of the imaging data obtained therefrom, is thus limited by the size of the second interior lumen 316. The size of the second interior lumen 316, however, could be increased by eliminating the first interior lumen 308.
Another concern with respect to intraluminal catheter assemblies is the coupling of an electrical signal between a distal non-rotatable operative element and a proximal rotatable operative element, such as, e.g., the ultrasonic transducer 306 and transceiver employed in the catheter assembly 300 described above. Typically, to provide this inductive coupling, an inductive coupler is connected in parallel with the signal wires at the proximal end of the catheter. As such, that portion of the signal wires distal to the inductive coupler rotate with the transducer, and must therefore be installed within the entire length of the drive cable. Although a proximally disposed inductive coupler adequately provides inductive coupling between the transducer and the transceiver, this arrangement has several disadvantages.
For example, a signal wire disposed drive cable aggravates a phenomenon suffered by ultrasound imaging catheters called non-uniform rotational distortion ("NURD"). NURD is caused by frictional forces between the rotating imaging core and the inner wall of the catheter, which are magnified by the many twists and turns that a catheter must undergo so that the transducer can be positioned in the desired imaging location within the patient's body. These frictional forces cause the imaging core to rotate about its axis in a non-uniform manner, thereby resulting in a distorted image.
NURD can be minimized by "optimizing" the construction of the drive cable, for example, by varying the drive cable's diameter, weight, material, etc. The characteristics of the drive cable, however, are dictated in part by the signal wires disposed therein, thereby limiting this NURD-minimizing optimization. Further, the signal wires contribute non-uniformities to the drive cable that cannot be optimized.
A further disadvantage of a proximally disposed inductive coupler is that the diameter of the drive cable must be increased to accommodate the signal wires, thereby occupying space within the catheter that could otherwise be used to support other functions such as, e.g., pull-wire steerability, angioplasty balloon therapy, ablation therapy, or blood flow (Doppler) measurements.
A further disadvantage of a proximally disposed inductive coupler is that the remoteness of the coupler prevents usage thereof for transducer optimization, i.e., transducer tuning and matching or prevention of transducer low frequency mode emittance. Thus, additional measures must be employed to either optimize the transducer or to minimize the undesirable effects thereof.
For instance, at its normal frequency of operation, the transducer exhibits a net capacitive reactance. Thus, inductive reactance should be provided to "cancel" this capacitive reactance, so as to efficiently couple the transmit/receive signals to the transducer (e.g., to maximize signal-to-noise ratios). A proximally disposed inductive coupler does not provide the needed inductance, however, since the inductance producing structure must be placed along the signal wires in close proximity to the transducer. Instead, such a result can be accomplished by placing an inductive coil in series with the signal wires, as demonstrated in U.S. Pat. No. 4,899,757 issued to Pope, Jr. et al.
In addition to canceling the capacitive reactance produced by the transducer, it is also desirable to match the input impedance of the transducer with the characteristic impedance of the signal wires, so as to minimize signal reflection. In particular, a proximally disposed inductive coupler is by definition proximal to the signal wires and can therefore not be used to perform such matching. An attempt can be made to optimize the size and material of the transducer for matching of the signal wires therewith. Such optimization is limited, however, and to the extent any signal reflections are not eliminated, the signal power will accordingly be reduced.
Still further, an excited transducer naturally creates a low frequency mode of vibration that further produces multiples of higher frequency modes (e.g., 4 MHz, 8 MHz, 12 MHz, etc.). These unwanted signals cannot be eliminated through the use of a proximally disposed inductive coupler, but must be filtered out at the proximal end of the catheter. The signals within the frequency band in which the imaging system is to be operated cannot be filtered out, however, and must be dealt with as interference.
Theoretically, a parallel inductor can be placed in close proximity to the transducer to short out the low frequency mode, thereby eliminating the higher frequency modes. Such an arrangement, however, is complicated and expensive, and thus inefficient for the mere purpose of eliminating unwanted modes of transducer vibration.
Therefore, it would be desirable to increase the available space within a catheter body by eliminating or at least reducing the number of interior lumens that support transmission lines. It would be further desirable to improve the mechanical and electrical performance of a catheter that employs a distal rotatable operative element and a proximal non-rotatable operative element.